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Not the person you replied to, but I'm guessing that they're a stickler for the original meaning, which is "to reduce by a 10th", hence the "deci" in decimate. It comes from a military discipline practice that could only have been invented by the Romans. The force to be punished is divided into groups of 10, and each group is given a bag containing 9 white pebbles and one black pebble. They each take turns reaching into the bag blindly and grabbing a pebble. The people with the white pebbles are then each given a cudgel and ordered to bludgeon the poor bastard with the black pebble to death, hence reducing their overall numbers by a tenth, or decimating them.